1. Field of the Invention
This invention relates to a misfire detection apparatus of an internal combustion engine for detecting combustion or a misfire of the internal combustion engine based on an ion current.
2. Background Art
It is generally known that if an air fuel mixture is burnt in a cylinder of an internal combustion engine, ions are produced. Then, if a probe with a high voltage applied as a bias voltage is installed in the cylinder, the ions can be observed as an ion current.
This means that the presence or absence of the ion current is detected, whereby it can be determined that combustion or a misfire occurs separately in each of all cylinders.
FIG. 6 is a drawing to show the configuration of a misfire detection apparatus of an internal combustion engine in a related art, and FIG. 7 is a timing chart to show the operation of the misfire detection apparatus. In FIG. 6, numeral 1 denotes an ignition section. The ignition section 1 comprises an ignition coil IG wherein a positive voltage VB is applied to the high-voltage side of a primary winding 11 and a switching element 13 for shutting off a primary current is connected between the low-voltage side and ground and an ignition plug 14 is connected to the high-voltage side of a secondary winding 12 and an ion current detection section 15 is connected to the low-voltage side via a wiring conductor.
The ion current detection section 15 includes a bias circuit 16 for applying a positive bias voltage to the ignition plug 14.
Next, the operation of the apparatus in the related art is as follows:
To use the secondary voltage of the ignition coil IG to detect an ion current, the bias circuit 16 charges the ignition plug 14 with a positive high voltage (bias voltage) as an ion current detection probe.
When an ignition pulse IB is given to the switching element 13, the primary current of the primary winding 11 of the ignition coil IG on the falling edge of the ignition pulse IB is shut off. A negative high voltage is applied to the ignition plug 14 connected to the secondary winding 12 and discharge is started between electrodes of the ignition plug 14, whereby an air fuel mixture is ignited and explosive combustion occurs. As explosive combustion of the air fuel mixture occurs, ions are produced by the ionization action in the explosive cylinder.
At this time, the positive bias voltage is applied to the ignition plug 14 by the bias circuit 16 charged by the secondary voltage and thus the ions cause an electron move to occur by the bias voltage and are detected as an ion current.
However, as shown in FIGS. 7A and 7B, before the ions are detected, a steep pulse occurs when the ignition pulse IB rises, and at the ignition time in the air fuel mixture caused by discharge by the ignition plug 14, a steep pulse also occurs as ignition noise when the ion current occurs.
Generally, the peak value of the ion current changes depending on the operation state of the engine; when the engine speed is low, the peak value tends to become small and when the engine speed is high, the peak value tends to become large. The value becomes several μA to several hundred μA. To detect a misfire based on the presence or absence of the ion current in all areas of the engine operation condition, the threshold value for ion current detection is set to several μA.
In fact, however, if the threshold value is set to several μA, when the ignition pulse IB rises, ignition noise occurring at the ignition time of the ignition plug 14 may be erroneously detected as a combustion pulse. Therefore, the pulses each having a narrow pulse width are eliminated by a mask circuit (mask means) 17 and then only the ion current component is pulse-shaped by a waveform shaping circuit 118 and the result is output as a combustion pulse.
Therefore, in the normal combustion, as shown in FIG. 7C, the combustion pulse provided by performing waveform shaping of the ion current by the waveform shaping circuit 118 is output in the mask time after the discharge starting.
FIG. 7D shows the ion current waveform at the misfire time; the pulse at the ignition pulse rising or falling time and the pulse caused by ignition noise are removed by the mask circuit and thus are not output. Therefore, as shown in FIG. 7E, combustion explosion in the cylinder is suppressed because of misfire, of course, and thus a combustion pulse caused by an ion current is not output either.
In the normal combustion state, as described above, the waveform of the ion current is shaped based on a predetermined fixed threshold value and whether or not a combustion pulse exists is determined, whereby whether the condition is combustion or a misfire can be determined. However, soot may be deposited between the electrodes of the ignition plug because of combustion of an air fuel mixture depending on the operation state of the internal combustion engine.
For example, assuming that the bias voltage is 100 V and that the insulating resistance of the ignition plug 14 when soot is deposited is 5 MΩ, a 20 μA leak current flows. Consequently, as shown in FIG. 8, as the ignition pulse IB is applied, while the leak current is monotonously attenuated with a predetermined time constant, it flows into the ion current detection section 15. At the discharge starting time of the ignition plug 14, an ion current caused by combustion is superposed on the leak current gradually monotonously decreased with time constant CR of high resistance caused by soot and capacitor component C of the bias circuit and flows.
Therefore, when the leak current flows, if the ion current input through the mask circuit is pulse-shaped by the waveform shaping circuit, the leak current of a predetermined pulse width generated by ignition pulse application, the leak current gradually monotonously decreased with the time constant CR is detected as a combustion pulse regardless of a misfire or combustion; this is a problem.